In the manufacture of fiber from molten material, it has been common practice to use electrically heated bushings made of precious metals including platinum, rhodium, palladium, ruthenium, iridium and alloys thereof. The bushings are electrically heated using the electrical resistance of the fiberizing bushings which are usually box-like, open on the top and comprise an orifice plate containing hundreds or thousands of orifices, with or without nozzles or tips welded or formed thereon, as shown by U.S. Pat. Nos. 4,207,086 and 4,078,413, which disclosures are hereby incorporated by reference.
In the manufacture of fiber from molten material, a fiberizing bushing is normally used such as those shown in U.S. Pat. Nos. 4,662,922, 5,935,291, 6,196,029, 6,408,654, 6,427,492, 6,453,702, 6,701,754 and 7,003,986, the disclosures of which are incorporated herein. By desired fiberization is meant that the bushing is operating making the desired fiber for the product being produced. To remove the heat from the meniscus and fiber that must be removed to cool the molten or plastic fiber so that it will have integrity and strength to endure the remainder of the process of making the fiber product, cooling members are located close to the orifices or nozzle tips. These cooling members can be either cooling tubes like shown in U.S. Pat. Nos. 3,628,930, 4,397,665, 5,244,483 and 6,196,029, the disclosures of which are hereby incorporated by reference, or cooling fins, sometimes called fin blades or fin shields, attached to a cooling manifold as are well known in the fiber industry.
To produce a fiber from a molten material, the molten material must be in a range of viscosity that will produce a fiber. If the viscosity of the material coming out of the tip is too high, stresses caused by attenuation in the meniscus will cause the fiber to break. If the viscosity of the molten material in the meniscus is too low, the surface tension forces in the meniscus trying to form a sphere from the molten material will break the fiber. Since the viscosity of the molten materials is always temperature dependent, it is extremely important that the temperature of the molten material exiting from all of the tips is safely within the fiberizing range. If the stress in the fiber at or near the bottom of the meniscus reaches a critical level, the fiber breaks.
When a fiber break occurs, the loose fiber soon causes other fibers to break and soon all, or most, fibers being formed beneath the bushing are broken, a stoppage of desired fiberization. This is called a “breakout” in the industry. After a breakout begins, it is necessary to wait a short time, usually tens of seconds up to a few minutes, for beads of molten glass to form beneath each bushing orifice or tip, and become large enough that they break loose and fall from the bottom of the orifice plate or tip pulling a very coarse fiber, called a primary fiber, onto the floor, into a scrap bin, basement or scrap bin beneath the forming room floor. This is normally called “beading out” in the industry and the “beading out” typically takes about 30-120 seconds. Once beaded out, or as soon as the operator is available, the operator or starting equipment can then restart a strand containing the primary fibers into a chopper or winder and again begin making the desired product.
When the bushing is running good product the fibers are moving away from the bottom of the bushing at a speed of thousands of feet per minute. This downward movement at this speed, of an array of hundreds or thousands of fibers, creates, due to friction between the air surrounding the fibers and the surface of the fibers, a partial vacuum (lower pressure zone) by pulling a stream of air downward. The partial vacuum causes a flow of cooling air from the surroundings into the array close to the orifice plate and tips of the bushing. This flow of inspirated air coming from outside the array of fibers cools the tips, meniscuses and the newly formed fibers somewhat. Much additional cooling is accomplished with the cooling members, cooling fins or cooling tubes, mounted beneath the bushing and close to the tips to cool the air tips and glass/fibers. The cooling of the bushing, tips and orifice plate, causes, with some delay, the temperature of the walls of the bushing to change or drop causing the power controller to send additional electrical power to the bushing automatically to maintain the set-point temperature. The method of controlling tip plate or orifice plate temperature, or bushing temperature, by using one or more temperature sensors located at one or more locations on the bushing walls performed satisfactorily on bushings of less than about 1600 tips and when other variables were less in control than today, but now with better control of parameters important to fiberization and particularly with much larger bushings of 6000 or more orifices and/or tips, better temperature control of the bushing orifice plate or tip plate and tips is needed, as indicated by the fiber diameter distribution of fiber product produced by both individual bushings and by a plurality of bushings.
When the bushing breaks out, this inspirated cooling flow of cooling air stops. At that time several more undesirable things begin. The set point thermocouple begins to heat up because of the loss of cooling air and as it does, the controller decreases the electrical power heating the bushing. As the electrical power is decreased during the beading out and hanging periods, the molten glass through-put decreases by 5-15 percent, slowing the flow of molten glass through the well, orifice, between the forehearth leg above the bushing and the bushing causing the temperature of the molten glass in the well, and thus the molten glass entering the bushing, to drop substantially, about 25-75 degrees F. This colder glass coming into the bushing causes the molten glass exiting the orifices to be colder and thus to have a higher viscosity. The higher viscosity glass has more resistance to attenuation when desired fiberization is restarted, causing higher stress in the fiber at its weakest point, and it frequently breaks. This is why the break rate is normally highest during the first ten minutes or longer after restart of desired fiberization, particularly as the area of the orifice/tip plate of the bushings has increased to accommodate mote orifices/tips. The larger the area of the orifice plate or tip plate, the greater the tendency to have a larger temperature variance across the orifice plate or tip plate or the tips. It normally takes about ten minutes or longer for the molten glass in, and exiting, the bushing to again reach the desired fiberizing temperature.
The above conditions apply to any molten material, but are most costly in the manufacture of so-called “continuous” glass fiber products from molten glass. In the manufacture of continuous glass fibers, melting furnaces are typically used to melt batch, refine the molten glass, and to feed molten glass through one or more forehearths and usually a plurality of bushing legs to the bushings. It is extremely important, to achieve a very low bushing breakout rate, that the molten glass coming to the bushings is fully melted and uniform in temperature and chemistry. Mixing in the molten glass is mainly dependent upon maintaining desired temperature gradients and flow patterns in the melting furnace. There are typically hundreds of thousands of pounds of molten glass, often about 500,000 pounds, in a typical melting furnace system for making continuous glass fibers. With this much molten material, the melting furnace and delivery system has great momentum and inertia, i.e. it is difficult and takes considerable time correct a change in the molten glass reaching the bushings following a furnace upset. A furnace upset is anything that makes a significant change in the way the melting furnace is operating, including a significant change in the throughput of molten glass through the delivery system, including the bushings. In the past it has been noticed that when a plurality of bushings were stopped from making desired fiber product and put into a hanging mode, to permit a chopper that had been pulling strands of fibers from the bushings to be rebuilt, that after a few minutes the conditions inside the melting furnace would change and that the automatic burner controls for the melter were changing conditions of the burners responding to the change(s) in the furnace. This happens on a smaller scale with every bushing breakout. This is necessary now, but not desirable. Although improvements in melting furnace control and stability have been made through the decades that large melting furnaces have existed, frequent furnace upsets or disturbances still exist result in lower productivity and higher manufacturing costs.
It is known that the end walls and sidewalls of electrically resistance heated fiberizing bushings made of a conductive material tend to be hotter in operation, causing the outer periphery of the tip plate of the bushing to also be hotter than the interior portion of the tip plate. Numerous modifications have been proposed and used to try to eliminate this problem, but it still exists even if at a lower temperature difference, particularly on bushings having about 2000 or more tips. Sometimes a generally center portion of the tip plate is also hotter during operation than the portions between the generally center portion and peripheral portion of the tip plate, being caused by hotter glass flowing into a center region of the bushing than the glass flowing into the regions surrounding the generally center region of the bushing.
It is also known to try to maintain a drawn-in cooling airflow into the tips and area beneath the bushing by starting a downward flow of high velocity air from an air tube mounted beneath the bushing as soon as a fiber breakout is detected. This helps, but does not fully compensate for the amount of air that is drawn-in during the desired fiberization operation. As a result the bushing heats up and the control system reduces the amount of power being passed through the bushing. Then when the bushing is restarted and the drawn-in air is once again flowing rapidly, fiber breaks often occur before the bushing can get back into desired fiberization equilibrium.
Attempts have been made to achieve uniform tip profiles across the tip plate of bushings by changing the sizes of holes in the bushing screen located above the tip plate. While this improves the uniformity, particularly of the molten glass reaching the tip plate, it is insufficient to overcome the problems of the peripheral tips breaking out much more often than the interior tips.